This week, scientists announced that for the first time ever, they were able to 3D print an organ, successfully transplant it into an animal and get it to work. If you're unsure of whether that's really as crazy as it sounds, it is.

For years scientists have succeeded at 3D printing "living" tissue, but that tissue has been too weak, too unstable and too small to implant into humans or animals. Getting the tissue to stay alive long enough to integrate with the body and fuse with its blood supply has been next to impossible. Because of those hurdles, some scientists are skeptical that printing organs for the human body will ever become more than science fiction.

The guiding principle behind 3D-printed organs is that if the right type of cell is placed in approximately the right spot, nature will take over, allowing different types of cells to arrange themselves and then fuse together on their own. Back in 2007, a Missouri professor printed out several types of chicken heart cells onto large sheets using a support gel to keep them in place. When the cells eventually arranged themselves and began beating, it ignited a race to print the first fully functioning human organ.

Using a 3D-printer, researchers "print" organs by using live cells as "ink" and layering them in a precise pattern, leaving them space to grow into fully formed organs. Set enough ear cells in approximately the right shape to make an ear and eventually, theoretically, you'll get one. Think of it like a very complicated game of connect-the-dots.

But living tissue is complex. Until now, tissue that was 3D printed usually died pretty quickly. Existing 3D printers couldn't create structures that were big enough or strong enough to support the tissue; it turns out it's tricky to get the soft, water-based gel embedded with cells to stay in place.

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The researchers at Wake Forest believe they have solved that problem. Their printing process includes a bio-degradable, plastic-like material that provides a strong, temporary outer structure to support the cells as they grow into formation. Researchers also optimized the "ink" that holds the cells with a latticework of tiny channels to allow nutrients and oxygen from the body to flow to the printed organ and keep it alive until it begins to form its own blood vessels.

According to Dr. Anthony Atala, the director of the Wake Forest Institute for Regenerative Medicine, the new printer could fabricate stable, human-scale tissue of any size or shape, using CT and MRI scans to create custom organs. “With further development," Atala said in a statement, "this technology could potentially be used to print living tissue and organ structures for surgical implantation."

According to the Wake Forest researchers' paper in Nature Biotechnology, they successfully printed ear, bone and muscle structures and then implanted them under the skin of mice and rats. Two months later, the ears had kept their shape and formed cartilage tissue. For the muscle, the researchers found that just a few weeks after surgery, the muscle implant had prompted nerve formation in rats. The bone implants, which were printed using human stem cells triggered the formation of a blood vessel system that was observable after five months: a new jaw bone implanted in a rat formed bone tissue.

Connecting 3D-printed tissue to working blood vessels in the body may be the most significant step yet toward the creation of functional artificial organs for humans. It means that artificial organs that could survive long term in the body are technologically feasible. The researchers told the Gulf News that their next step is to print more complex organs.

"They were able to get large constructs that were viable long enough to be implanted, which is not trivial at all," Gordana Vunjak-Novakovic, a biomedical engineer at Columbia University, told The Verge. "This is an important study that shows, convincingly and elegantly, that custom-designed tissues can be produced in lab."

But 3D-printed organs aren't quite ready for human transplant. For one, the parts need to be monitored longer, to make sure they do not eventually die or decay and to see how well they perform in the body over time.

The promise of 3D printing isn't just creating a new liver for that person who has been waiting for one for years on the national transplant list. It's creating tailor-made organs the right size and shape for a person's body, grown from a person's own cells so that the body doesn't reject it.

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Jennifer Lewis, a bioengineer at Harvard University, recently told Nature that she was skeptical that 3D-printed livers and kidneys could ever become a reality.

“I would love for that to be true,” she said. “But these are highly complicated architectures.”